1. Field of the Invention
The present invention generally relates to a method of forming a fine resist pattern, and more specifically, it relates to a method of forming a fine resist pattern which is improved to be capable of forming a fine resist pattern exceeding the performance of an apparatus therefor. The present invention also relates to a postexposure baking apparatus which can implement such a method.
2. Description of the Background Art
In relation to photolithography, a chemical amplification resist is now being proposed as a material having high sensitivity and high resolution. The feature of this resist resides in that a pattern forming reaction progresses through baking after exposure (hereinafter referred to as postexposure baking) with a catalyst of an acid which is generated during the exposure, to complete pattern imaging.
FIGS. 27 to 31 are sectional views of a semiconductor device showing respective steps of a conventional method of forming a fine resist pattern.
As shown in FIG. 27, a negative chemical amplification resist 1 (hereinafter referred to as a resist 1) is formed on a wafer 2. The resist 1 contains a base resin which contains hydroxyl groups, an acid generator which generates sulfonic acid by irradiation with an electron beam, and a cross-linking agent which reacts with the hydroxyl groups contained in the base resin by a catalytic action of a proton of the sulfonic acid, thereby bridging the base resin. In more concrete terms, the base resin is prepared from novolak resin shown in FIG. 32A, the cross-linking agent is prepared from hexamethylolmelamine hexamethylether shown in FIG. 32B, and the acid generator is prepared from triphenylsulfonium triflate shown in FIG. 32C respectively.
As shown in FIG. 28, the chemical amplification resist 1 is selectively irradiated with an electron beam 4 of 20 KeV in energy at an exposure value of 1 to 10.times.10.sup.-6 C/cm.sup.2, and separated into an exposed part 51 and unexposed parts 50. In the exposed part 51, the triphenylsulfonium triflate decomposes along a reaction formula shown in FIG. 33, to generate trifluoromethanesulfonic acid which is a strong acid.
Then, a hot plate 11 is employed for carrying out baking after exposure (postexposure baking: hereinafter referred to as PEB) at 50.degree.to 150.degree. C. for 2 minutes, as shown in FIG. 29. The state of bridging of the base resin in the exposed part 51 of the resist 1 which is caused by this PEB is now described with reference to FIGS. 34(a) to 34(e).
Referring to FIG. 34(a), the chemical amplification resist 1 is irradiated with the electron beam 4. Upon such irradiation of the chemical amplification resist 1 with the electron beam 4, the triphenylsulfonium triflate decomposes to generate an acid (H.sup.+), as shown in FIG. 34(b). When this resist 1 is baked, the base resin is partially bridged through the acid (H+) serving as a catalyst as shown in FIG. 34(c), to by-produce the acid (H.sup.+). The base resin is further chain-reactionally bridged successively through the by-produced acid (H.sup.+) serving as a catalyst, as shown in FIG. 34(d). Due to this chain reaction, the base resin is bridged in the form of a network as shown in FIG. 34(e). FIG. 35 shows reaction formulas of the aforementioned chain reaction.
As shown in FIG. 30, the resist 1 is baked to be separated into a bridged part 1a and unbridged parts 1b. Referring to FIGS. 30 and 31, the resist 1 is exposed to form a fine resist pattern (1a).
As hereinabove described, the chemical amplification resist 1 is baked after the exposure so that a pattern forming reaction progresses through the acid which is generated during the exposure for serving as a catalyst, to complete pattern imaging. Therefore, the pattern shape is extremely changed by diffusion of the acid as generated in the resist 1. Further, an alkaline substance contained in the air may adhere to the surface of the resist 1 or the acid as generated is neutralized and deactivated by alkali coming from the underlayer (2), to extremely change the pattern shape.
In order to prevent such change of the pattern shape caused by diffusion or deactivation of the acid, the PEB is generally carried out immediately after the exposure, to thereafter perform development. Referring to FIGS. 28 and 31, an exposed dimension D.sub.1 and a finished dimension D.sub.2 are equal to each other when the PEB and the development are carried out immediately after the exposure. In this case, the exposed dimension D.sub.l is the finished dimension D.sub.2 as required, and hence it is impossible to form a fine pattern exceeding the performance of an apparatus therefor. In other words, a pattern which is finer than the minimum resolution cannot be formed in the prior art.
In the aforementioned conventional method, further, the time margin for the process is extremely small since the PEB must be immediately carried out after the exposure.